Obesity is a health concern in horses worldwide, with a prevalence estimated between 31 – 45% in some equine populations. 
Among ponies and easy keeper breeds, the prevalence is even higher with one study reporting that 72% of adult ponies were overweight or obese. 
Horses are generally considered obese when they have a body condition score of greater than 7 on the Henneke 9-point scoring system. 
Obesity is a welfare concern for horses because it increases the risk of other health conditions including laminitis, metabolic syndrome, arthritis, respiratory problems and poor fertility. Obesity can also negatively impact your horse’s performance and soundness. 
While many factors can influence obesity, horses become overweight when they consume more energy than they expend on activity and maintenance over a long period of time. 
In this article, we’ll explore the causes, consequences, and risk factors for obesity in horses. If your horse is obese, speak with your veterinarian and your equine nutritionist to formulate a plan for healthy weight loss.
Is my Horse Obese?
Obesity is defined as the excessive accumulation of body fat resulting in a negative impact on the horse’s health. 
A horse is considered obese when fat accounts for more than 20% of its total bodyweight. Some very obese horses may have a body fat percentage of up to 40%. 
Judging whether your horse is overweight or obese can be challenging; research shows that horse owners frequently underestimate how fleshy their equine companions are. 
The 9-point Henneke Body Condition Scoring system can be used to evaluate whether your horse is overweight or obese.
This scale assesses your horse’s body condition by observing and palpating specific areas where fat accumulates in the body. These areas include the crest of the neck, the withers, behind the shoulders, over the rib cage, on the rump, and above the tailhead. 
You can determine your horse’s body condition by printing out Mad Barn’s Equine Body Condition Score Worksheet.
Risk Factors for Equine Obesity
Any horse can become overweight or obese; however, some horses are more prone to gaining weight than others.
- Feed provision
- Breed & Genetics
- Injury or stall rest
Providing more calories than the horse needs over a long period leads to fat accumulation, weight gain and obesity.
Overfeeding results in a positive energy balance and can occur if your horse:
- Is fed too much feed
- Is consuming energy-dense feed
- Has unrestricted access to lush pasture
- Is provided forage that is higher quality than required
Most of your horse’s diet should be fibre-rich forage. The quality of your horse’s forage should also match their energy needs.
Mature horses at maintenance (not exercising) can maintain a healthy body condition with relatively low-quality grass hay. These horses may consume too many calories if provided high-quality forages, such as alfalfa or immature grass hay.
In comparison, horses with higher energy needs such as performance horses or lactating mares require higher-quality forages to maintain their body condition.
Breed & Genetics
Some equine breeds are more prone to obesity than others. These breeds are generally identified as easy-keepers or good-doers because they tend to gain weight easily and require less feed to maintain body condition. 
Cob-type horses and native pony breeds are at the greatest risk of obesity. Cobs are 13.6 times and ponies are 2.3 times more likely to be obese than lightweight breeds. 
Drafts and Quarter Horses have the next highest risk of being overweight, followed by Tennessee Walking Horses. Lightweight breeds such as Arabians and Thoroughbreds have a lower risk of obesity. 
Certain breeds are more susceptible to becoming obese because of their genetics. Several traits linked to obesity such as cresty neck and insulin resistance have a genetic component in horses.
For example, a study in Welsh ponies and Morgan horses prone to obesity found a higher proportion of genetic variants linked to inflammation and dysregulated sugar and fat metabolism. 
The predisposition to obesity may be an evolutionary adaptation to poor-quality forages found in the wild.
When these breeds are fed improved high-quality forages, grains or commercial feeds they extract an abundance of calories, leading to weight gain.
Horses considered to be more dominant within the social hierarchy of their herd are at greater risk of obesity. 
Dominant horses have better access to communal food sources, such as hay feeders, and may consume more calories than their less dominant counterparts.
Horses that are higher in the herd hierarchy are also given more space to graze when turned out in a group. In comparison, low-ranking horses spend more time and energy being chased away or moving around dominant horses and may have a higher stress level.
Horses over 4 years old are at a greater risk of becoming overweight. 
Younger horses expend more energy growing and are more active than older horses, which could explain the difference in obesity risk between age groups. 
Historically, senior horses were likelier to be underweight than overweight. Horses over the age of 20 have more dental issues, exercise intolerance, and metabolic dysfunction that influences their ability to consume and expend calories. 
However, with modern management practices including regular deworming, dental and veterinary care, senior horses are better able to maintain a healthy body condition.
In recent surveys in North America, Europe, and Australia, senior horses are now almost equally likely to be overweight as underweight. 
Horses with a heavier workload have a lower risk of becoming overweight because of their increased energy expenditure. The more activity that your horse gets, the less likely they are to consume more calories than they are burning.
Horses ridden for pleasure have a lower risk of becoming overweight, and competition horses or horses in intense exercise routines have an even lower risk of obesity. 
Exercise also helps to improve insulin sensitivity, which contributes to healthy weight management. Seven days of light exercise improve insulin sensitivity in obese horses by as much as 60%. 
The management of horses encompasses everything from housing to daily routine, feeding, socialization, training, veterinary care, and more.
Management factors can impact your horse’s obesity risk by affecting feed intake, nutrient utilization and energy expenditure.
Several survey studies have identified management risk factors for obesity. One study found that larger, professional horse-keeping facilities had fewer overweight horses. 
Larger facilities are likely more knowledgeable about feeding, exercising, and maintaining an ideal weight on horses. Weight maintenance may also be more important to professional facilities with horses used in shows or competitions.
Another survey found that horses living on farms raising non-equine livestock for meat were more likely to be overweight.  This may be attributed to different feeding practices.
It is quite normal for horses to lose body condition throughout the winter because there is less available forage and the horse’s maintenance energy requirements increase due to thermoregulation. 
Sugar and starch levels in grass are highest in the spring when the grass is lush and growing quickly. This can contribute to weight gain in horses as summer approaches. 
Sugar and starch are non-structural carbohydrates (NSC), which are quickly digested in the foregut and provide the horse with rapidly available energy. In excess, non-structural carbohydrates contribute to weight gain.
Injured horses are prone to weight gain due to reduced activity and lower energy expenditure. 
If your horse is injured and put on stall rest or assigned a lower workload, consult with an equine nutritionist to adjust their feeding plan. Feed and pasture must be reduced in proportion to the decrease in exercise to prevent weight gain.
Negative Effects of Obesity in Horses
Horses become obese from consuming more energy than they expend. These extra calories contribute to fat accumulation and cause physiological changes that are generally detrimental to equine health.
Insulin Levels and Glucose Storage
When horses consume feed containing glucose (derived from starch or simple sugars), blood glucose levels rapidly rise. This triggers insulin secretion from the pancreas.
Insulin is a hormone that regulates blood sugar levels by promoting glucose uptake into cells. Insulin also tells the body to store glucose as glycogen or fat until the horse needs it for energy.
In some horses, prolonged intake of high carbohydrate feeds such as grains can lead to impaired insulin sensitivity. This means that the horse’s cells do not effectively respond to the presence of insulin. Less glucose is removed from the blood and stored as glycogen or fat. 
Horses with low insulin sensitivity are described as insulin-resistant. Insulin resistance is problematic because glucose builds up in the blood causing hyperglycemia, or high blood sugar. 
In response to hyperglycemia, the pancreas produces more insulin to remove extra glucose from the blood. Hyperinsulinemia or high insulin in the blood can contribute to complications such as laminitis. 
Altered Fat Metabolism
Horses and other mammals convert excess glucose into glycogen, which is made of long chains of glucose molecules. Glycogen is a rapidly available form of energy stored in the liver and skeletal muscle.
Glucose is also stored as fat molecules in adipose tissue. Adipose tissue can be found throughout the body but mainly under the skin (subcutaneous adipose) or around organs within the body (visceral fat).
The cells of adipose tissue are designed to store large amounts of fat. However, in obese animals, these cells might be unable to keep up with energy storage demands.
This can lead to spillover in which fat is no longer just stored in adipose tissue, but also gets stored in the liver and muscle. Because these tissues are not as well equipped to store fat, this can induce inflammation and directly cause insulin resistance. 
A blood test in an overweight or obese horse may reveal issues with fat metabolism such as:
- High levels of non-esterified fatty acids released by adipose cells
- High levels of VLDL, LDL and HDL cholesterol produced by the liver
- Abnormal liver enzyme levels suggesting damage from fat accumulation in the liver
As adipocytes (adipose tissue cells) expand in size to store more fat, they experience stress. This causes the cells to release proinflammatory molecules known as cytokines.
Cytokines are proteins including TNF-alpha, interleukin-1, interleukin-6, and interleukin-8. They are responsible for upregulating the immune system and inducing inflammation. 
The release of cytokines activates immune cells within adipose tissue to support stressed cells and remove waste products that may be produced. However, these proinflammatory cytokines can also induce inflammation elsewhere resulting in systemic inflammation.
Inflammatory cytokines disrupt insulin signalling which can cause or worsen insulin resistance. 
Adipocytes (fat cells) produce signaling molecules known as adipokines, including the hormones adiponectin and leptin. 
Adiponectin is produced by adipocytes to promote insulin sensitivity and reduce inflammation. 
Leptin is a chemical messenger that helps to control appetite and promotes activity to maintain body condition. Leptin release is triggered by feed intake.
When your horse consumes calories, leptin sends a message to the brain that the body is in positive energy balance. This results in feelings of fullness and appetite suppression. 
Overweight horses can also get hyperleptinemia (high blood leptin) and become leptin resistant. Like insulin resistance, leptin resistance occurs when the brain stops responding to the presence of leptin and the body produces more of this hormone to compensate. 
In leptin-resistant horses, the absence of leptin signalling makes it even easier for overweight horses to gain excess weight. 
- Vitamin E
- LDL, VLDL, and HDL cholesterol
- Non-esterified fatty acids (NEFAs)
In contrast, adiponectin levels decreases in the blood as body mass increases. Adiponectin is the hormone produced by fat cells to combat inflammation and improve insulin sensitivity. 
Your veterinarian may assess some or all of these factors in your horse. Tracking these levels over time can help assess whether your horse’s metabolic health is improving or worsening.
Health Consequences of Obesity in Horses
Horses that carry excess body condition are at risk of many health conditions, including: 
- Equine metabolic syndrome
- Insulin resistance
- Glucose intolerance
- Hyperinsulinemia (elevated insulin)
- Systemic inflammation
- Hepatic lipidosis
- Osteochondrosis (developmental disease)
- Impaired reproductive performance
- Osteoarthritis (degenerative joint disease)
- Exercise intolerance
- Heat intolerance
- Hyperlipemia and dyslipidemia
Equine Metabolic Syndrome
Obesity is associated with Equine Metabolic Syndrome (EMS), a condition which is similar to Type 2 diabetes in humans.
EMS is characterized by several metabolic changes in the horse including hyperglycemia (high blood glucose), hyperinsulinemia (high blood insulin), and insulin resistance (the body is less responsive to insulin). 
General obesity may be associated with just one or multiple of these metabolic alterations. 
Horses with EMS are also likely to have excessive regional fat deposits in specific areas such as the crest of the neck or prepuce.
Metabolic syndrome also puts horses at high risk of developing laminitis. 
The coffin (pedal) bone inside the horse’s hoof is connected to the hoof wall by soft tissue structures known as laminae.
In horses with laminitis, the laminae become inflamed and in severe cases can break down. This causes the coffin bone to loosen from the hoof wall and rotate, putting pressure on the sole of the horse’s foot and resulting in severe pain. 
Researchers have not yet determined whether the increased risk of laminitis is attributed to the greater pressure on the hooves caused by the horse’s excess weight causes or if other conditions caused by obesity are responsible. 
Obesity can also lead to EMS, hyperinsulinemia, and inflammation, each of which is a risk factor for laminitis. 
Pituitary Pars Intermedia Dysfunction (PPID)
Pituitary Pars Intermedia Dysfunction (sometimes referred to as Equine Cushing’s Disease) is a neurodegenerative condition that affects hormones produced by a specific part of the brain called the pituitary gland.
Symptoms include changes in behaviour, cresty neck, muscle loss, and growth of a thick winter coat that is slow to shed out.
Approximately one-third of horses with PPID have insulin resistance. These horses may have previously been overweight or obese.
However, as the condition progresses, weight loss and loss of muscle mass are common. In some horses with PPID, insulin resistance persists putting them at higher risk of laminitis. Cresty neck may also be observed in horses with PPID. 
Overweight horses are at risk of hepatic lipidosis or fatty liver syndrome, a state in which fat accumulates in the liver and impedes liver function. 
The liver plays an important role in metabolism, detoxification and immune function. If excess fat accumulation impairs liver function, the horse can experience significant complications. 
Hepatic lipidosis most common affects ponies, donkeys, and miniature horses. 
Osteochondrosis affects young, growing horses and is characterized by abnormal cartilage growth on the ends of bones where they form a joint with other bones. Severe cases of osteochondrosis can cause joint inflammation or even lameness in young horses.
Growing horses that are fed excess calories to promote fast growth are most at risk of becoming overweight and developing DODs. 
Obese horses may have fertility issues and reduced reproductive efficiency. Time between ovulations is longer in obese mares than in feed-restricted mares, resulting in fewer breeding opportunities.
The hormone progesterone remained elevated for longer than normal in overweight mares, suggesting that mechanisms for triggering ovulation are suppressed in obese mares. 
However, horses with a body condition score below 5 also have a reduced pregnancy rate and are less likely to maintain pregnancy to term. Keeping broodmares at the optimal weight of 5 or 6 is important for fertility and performance. 
Other Impacts of Obesity
Obesity in horses often coincides with elevated consumption of carbohydrates. High-carbohydrate diets can lead to gastrointestinal issues such as hindgut acidosis, altered microbiome, and increased gut permeability. 
These changes in gut function have been linked to inflammation in the body and laminitis. 
Excess body fat also acts as an insulating layer and can interfere with thermoregulation – the horse’s ability to regulate their core temperature. This could lead to heat intolerance and poor performance in hot conditions.
The impact of obesity on thermoregulation has not been studied in horses and is poorly understood in most mammals. However, excess fat makes it more difficult for the body to dissipate heat in humans, putting overweight humans at higher risk of heat stress. 
Supporting Weight Loss in Obese Horses
If your horse is obese or overweight, work with your veterinarian and equine nutritionist to formulate a feeding and management plan to support healthy weight loss.
- Start by assessing your horse’s body condition and current activity level to determine their energy requirements and how much weight they need to lose.
- Evaluate your horse’s current feeding program to calculate how much digestible energy is being provided. Overweight horses need lower energy in their diet to create a calorie deficit.
- Remove or reduce high-calorie commercial feeds, supplemental fat sources and concentrates such as grains or beet pulp in your horse’s diet.  Choose lower-quality forages and consider soaking your hay to reduce sugar and starch content.
- If removing grain and concentrates does not allow your horse to reach an ideal body condition, limit forage to 1.4-1.7% of the horse’s body weight per day.  A slow feeder or a hay net is recommended to extend your horse’s foraging time.
- Avoid over-restricting feed intake, which is a risk factor for hyperlipidemia.
- Ensure that your horse’s vitamin and mineral requirements are still being met. Feed a concentrated vitamin and mineral supplement such as Mad Barn’s Omneity to support metabolic health and overall well-being.
- If your horse is sound, implement an appropriate exercise routine to increase your horse’s energy expenditure and improve insulin sensitivity.
Regularly assess your horse’s body condition to monitor progress and determine whether further dietary changes are needed to reach your horse’s goal weight
For additional tips on feeding and managing obese horses, submit your horse’s information online to receive a free diet evaluation from our qualified equine nutritionists.
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